From: Tom Lane <tgl@sss.pgh.pa.us>
To: Craig Ringer <ringerc@ringerc.id.au>
Cc: McKinzie, Alan (Alan) <alanmck@avaya.com>
Cc: pgsql-performance@postgresql.org <pgsql-performance@postgresql.org>
Subject: Re: Are there known performance issues with defining all Foreign Keys as deferrable initially immediate
Date: Sun, 16 Sep 2012 11:37:15 -0400
Message-ID: <11998.1347809835@sss.pgh.pa.us> (raw)
In-Reply-To: <5055DE3D.4040603@ringerc.id.au>
References: <E151F470685C15418AEA609E825A171022EC8EA823@DC-US1MBEX4.global.avaya.com>
<5055D7FF.2070908@ringerc.id.au>
<5055DE3D.4040603@ringerc.id.au>
Craig Ringer <ringerc@ringerc.id.au> writes:
> Found it, it's in the NOTES for CREATE TABLE.
> http://www.postgresql.org/docs/current/static/sql-createtable.html:
> When a UNIQUE or PRIMARY KEY constraint is not deferrable, PostgreSQL
> checks for uniqueness immediately whenever a row is inserted or
> modified. The SQL standard says that uniqueness should be enforced only
> at the end of the statement; this makes a difference when, for example,
> a single command updates multiple key values. To obtain
> standard-compliant behavior, declare the constraint as DEFERRABLE but
> not deferred (i.e., INITIALLY IMMEDIATE). Be aware that this can be
> significantly slower than immediate uniqueness checking.
Note that that is addressing uniqueness constraints, and *only*
uniqueness constraints. Foreign key constraints are implemented
differently. There is no equivalent to an immediate check of a foreign
key constraint --- it's checked either at end of statement or end of
transaction, depending on the DEFERRED property. So there's really no
performance difference for FKs, unless you let a large number of pending
checks accumulate over multiple commands within a transaction.
regards, tom lane
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To: pgsql-performance@postgresql.org
Cc: tgl@sss.pgh.pa.us, ringerc@ringerc.id.au, alanmck@avaya.com
Subject: Re: Are there known performance issues with defining all Foreign Keys as deferrable initially immediate
In-Reply-To: <11998.1347809835@sss.pgh.pa.us>
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